The present invention relates generally to a control system for a plurality of rappers in an electrostatic precipitator. More particularly, the invention relates to such a system which controls a plurality of rappers in a desired sequence, which controls the rapping intensity of each individual rapper with enhanced accuracy, and which has provisions for indicating either an open circuited or a short circuited condition in any individual rapper or electrical connection thereto.
Electrostatic precipitators are widely employed, particularly among industrial users, for removing particulate from gases. A typical large electrostatic precipitator includes a housing in which banks of vertically-extending collecting electrode plates or curtains are disposed, with particulate-laden gas passing through the housing parallel to the the plates. The particulate carried by the gas stream is charged to one polarity by means of a corona discharge, and the collecting electrode plates are oppositely charged. The charged particles are therefore electrostatically attracted to the collecting electrodes.
In order to remove the collected particulate from the collection electrodes, rapping or vibrating devices are commonly employed. In a large precipator, there are a plurality of individually controlled rappers, each rapper vibrating an electrode group comprising one or more electrode plates. Collected particulate is dislodged by the vibration and falls by gravity to a sump or the like for removal. In such a system, to prevent noticeable reentrainment of collected particulate, it is desirable to operate only one rapper at a time. Further, it is known to be highly desirable to be able to control the rapping intensity of each individual rapper in the system. Various sections of a large precipitator tend to collect particulate at different rates. If rapping intensity higher than necessary for the actual level of particulate buildup in a particular section is employed, unnecessary stress is applied to the mechanical elements of the precipitator, leading potentially to premature failure.
A typical electromechanical rapper comprises a vertically movable plunger biased downwardly, for example by gravity, towards an impact and resting position. Preferably, the plunger rests upon an anvil rigidly connected to a group of collection electrode plates. For displacing the plunger, an electromagnetic coil is provided, which, when energized, lifts the plunger to a desired height. When the electromagnetic coil is subsequently de-energized, the plunger falls, striking the anvil and imparting vibration to the connected collection electrode plates. Rapping intensity accordingly depends upon the plunger displacement or lift before release. Plunger lift, and therefore rapping intensity, may generally be controlled by controlling the energy applied to the rapper coil.
One example of an electrostatic precipator rapper control system is disclosed in a commonly-assigned U.S. Pat. No. 3,504,480--Copcutt et al. The Copcutt et al control system generally addresses the concerns mentioned above. Power is sequentially fed to a plurality of rappers by a distribution switch. In order that the rappers may operate at different controlled intensities, power is supplied to the rappers through conduction-angle-controlled SCR's. In the Copcutt et al system, the intensity of each rapper is separately controlled.